What is a Virus – Definition

Virus – an infectious pathogen that must invade a host cell to replicate, technically called an obligate intracellular parasite. A virus is a particle of living material that contains an inner core of nucleic acid (DNA or RNA), called the genome, encased in an outer shell of protein, called a capsid.

Some viruses contain a third layer composed of lipids, called an envelope, that further protects and nourishes the virus. These components, collectively called a virion, cannot themselves support a full life cycle, which obligates the virus to find a host to maintain its survival.

A virus can attach only to the type of cell capable of supporting it, binding to specific protein molecules on the surface of the cell membrane.

How Viruses Work

After invading a host cell, a virus hijacks the cell’s structures and functions to serve its own needs and to replicate itself. DNA viruses produce proteins that the host cell’s RNA transcribe as instructions to replicate the virus’s DNA, which the cell does. DNA contains the instructions for the cell’s functions; RNA forms the messenger proteins that carry out the directives of the DNA.

Eventually the virus’s copies of DNA crowd out the cell’s copies of DNA, and the cell becomes the agent of the virus. The cell either divides or ruptures, spreading the virus. RNA viruses achieve a similar result by causing the host cell to replicate their RNA, which then replaces the cell’s RNA.

Retroviruses are RNA viruses that contain the enzyme reverse transcriptase, which allows RNA to instruct DNA (the reverse of normal).

Virus Adaptability

Viruses are highly adaptable and have numerous mechanisms to hide from the immune system, allowing them to become well established infections before the immune system detects their presence. Once the immune system does detect a virus, it develops antibodies that protect against subsequent infection by the same virus.

Many common viruses-such as those that are responsible for colds (rhinoviruses), gastroenteritis (enteroviruses), and the flu (influenza viruses)- frequently alter their structures, evolving into different strains that can cause the same illnesses.

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